Attention has been focused on a technique for forming a transistor using semiconductor thin film formed over a substrate having an insulating surface. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). As a semiconductor thin film applicable to the transistor, a silicon-based semiconductor material is widely known; moreover, an oxide semiconductor has been attracting attention as another material.
For example, a transistor whose active layer uses an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Document 1).